The present invention relates to medical imaging, and particularly, to the reduction of motion artifacts in images produced using a projection method of reconstruction.
There are a number of modalities used to produce medical images. These include x-ray computed tomography (CT), magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT), and positron emission computed tomography (PET) methods. In all cases, the data used to reconstruct the desired image are acquired over a period of time in a scan comprised of a series of projections. Each projection is a snapshot of the patient from a different angle, or perspective, and a scan typically includes tens, or hundreds of projections. In the case of x-ray CT the entire data set may be acquired in a few seconds, whereas an MRI scan typically requires a few minutes to complete. The methods used to reconstruct an image from such data sets presume that the patient is motionless during the entire scan and that the same fixed object is the subject of all acquired projections. To the extent this is not true, artifacts such as ghosts, smearing and fuzziness appear in the reconstructed image.
Efforts to reduce patient motion during a scan can significantly improve image quality. However, artifacts caused by respiration are a significant problem in chest scans where suspension of breathing is not possible or poor instructions are provided to the patient by the scanner operator. Children and comatose patients are routinely scanned with no attempt to synchronize respiration with scanning, and it is expected in such cases that a number of poor quality images will be produced and will be discarded.
One approach to reducing motion artifacts in medical images is to retrospectively correct the acquired data to offset the effects of motion. One such method, for example, is disclosed in U.S. Pat. No. 4,937,526 and is applied to acquired MRI data. The corrections that are made may be determined from an examination of the acquired raw data itself, or additional information, such as a signal from a cardiac monitor or a respiration monitor, may be used. The manner in which the corrections are made to the acquired raw data is determined by the particular reconstruction technique that is used. In the above patent, for example, a 2D Fourier transformation is used to reconstruct an image from the acquired MRI data, and the correction methods disclosed are limited to that technique.
The back projection method for image reconstruction is employed to some extent in all computed medical imaging modalities. It is the predominant method used in x-ray CT, and there is a need to correct acquired data used in projection imaging for the effects of patient motion.
The related application Ser. No. 07/615,778, now U.S. Pat. No. 5,121,128, referred to above, provides a method of modifying the back projecting process to accommodate motion of the patient occurring during acquisitions of the projections. In particular, the patient motion is modeled as a two dimensional magnification and offset of the volume elements (voxels) of the patient such as might occur with expansion of the chest during breathing.
The possibility of the above modification to the back projecting process was founded on an analyses of the image reconstruction process as a Fourier transform using the Fourier slice theorem. The results of this analyses were then applied to the more typically used backprojection process. The results suggested that the modification was appropriate provided the motion could be modeled as a simple, global magnification and offset of the voxels of the patient. Nevertheless, in general, the patient motion does not conform to a simple magnification and offset of the patient voxels but is a more complex function of time.